Quarterly Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues	$ 495	$ 429	$ 424	$ 404	$ 457	$ 421	$ 418	$ 403	$ 1,752	$ 1,699	$ 1,645
Operating Income (Loss)	58	46	20	(23)	58	60	5	(14)	101	109	109
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 131	$ 15	$ (37)	$ (52)	$ 8	$ 54	$ (29)	$ (37)	$ 57	$ (4)	$ 69
Earnings Per Share, Basic	$ 1.02	$ 0.11	$ (0.28)	$ (0.41)	$ 0.06	$ 0.42	$ (0.23)	$ (0.29)	$ 0.44	$ (0.03)	$ 0.52
Earnings Per Share, Diluted	1.01	0.11	(0.28)	(0.41)	0.06	0.42	(0.23)	(0.29)	0.44	(0.03)	0.51
Common Stock, Dividends, Per Share, Declared	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 1.00	$ 1.00	$ 1.00
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Provisional Income Tax Net Expense (Benefit)	$ 183
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Per Diluted Share	$ 1.39